The present invention pertains to boro-organic synthesis and more particularly to the methylation of boron atoms in carboranes.
The substitution of methyl groups on the boron atoms of carboranes is advantageous in that polymers derived from B-methylated carboranes often have improved properties. For example the B-methyl derivatives of 2,4-dicarba-closo-heptaborane-7 exhibit high thermal stability and thus would provide excellent high temperature elastomers.
The only method presently being used for methylating carboranes involves pyrolysis of a nido-carborane in the presence of excess trimethylborane. This method is unsatisfactory on account of the low yields. Various B-methylated derivatives have been produced as a by-product in the reaction of pentaborane-9 with acetylene in a flow system, but the yields are extremely small -- trace amounts only. Attempts to methylate carboranes by sequentially reacting the carborane with bromine and lithium methyl proved to be completely unsuccessful. The anticipated reaction was the removal of the bromine by formation of lithium bromide salt followed by attachment of the methyl group to the vacated boron atom. Instead, however, the bromine atom remained attached to the boron and the lithium atom substituted on the carbon atom.